1. Field of the Invention
This invention relates to addition and replenishment of electrolyte to a fuel cell stack under controlled conditions, and more particularly to addition and replenishment of a liquid electrolyte in a manner which alleviates flooding of the fuel cell electrodes.
Each of the individual cells forming the fuel stack utilizes an electrolyte which is intended to fill up a porous matrix for the saturation thereof and to keep the porous matrix saturated as well as to keep the remaining area around the matrix wetted to maintain the matrix in a state of electrolyte-wetted condition.
The invention provides for an acid channel arrangement which permits acid feed to a large number of cells without producing a large hydrostatic head which could cause flooding of electrodes. A series of back and forth flow arrangements distributes the overall head generally uniformly between cells due to frictional pressure drop in the channel for each cell.
With any fuel cell stack, the porous matrix of the fuel cell must be saturated with acid electrolyte to transport ions and to provide a seal which prevents mixing of the reacting gas streams. Even if wet matrix assembly procedures are used, acid must be added after some period of operation to replace losses by evaporation to the gas streams.
2. Description of the Prior Art
The prior art in general has been discussed in copending application Ser. No. 303,809. However, for the sake of completeness, the disclosure of the aforesaid application is incorporated by reference. Nevertheless, it should be noted that in the general prior known fuel cell stacks, acid in an acid fuel cell having two electrodes is contained in a thin porous member which is referred to as the matrix. The matrix is positioned between the anode and cathode electrodes, which in turn are positioned between a pair of bipolar plates; for purposes of description, the anode-matrix-cathode forms a first sandwich construction and the two bi-polar plates with the first sandwich construction therebetween forms a second sandwich construction. Once the cell is assembled and forms part of the fuel stack, acid additions to the matrix form an acid reservoir channel. There are many presently known methods and systems.
In accordance with a heretofore known method and system of adding electrolyte or acid to the fuel cell stack, acid addition utilizes acid channels in the bipolar plates which supply acid to one edge of the matrix of each cell in the stack. In order to apply the same hydrostatic head of acid to each matrix, the stack must be arranged with the bipolar plates in a vertical plane with acid at the bottom edge of the plates. Wicking must then be accomplished against the gravity feed which results in a slow wicking process. Structurally, it is desirable for tall stacks to be operated with plates in a horizontal plane. Thus re-wicking requires removal of the stack from operation to lay it on its side.
The presently used acid addition channel configuration is shown in FIGS. 1 and 2, and is labelled as prior art. The portion of the bipolar plate with acid channels and plate to plate holes is shown in FIG. 1. The stack arrangement is shown in FIG. 2.
U.S. Pat. No. 3,926,676 to Frie et al. is concerned with a uniform supply of electrolyte to all fuel cells of a battery. Electrolyte is supplied through inlets at the bottom of the battery and pumped upwardly to the top and out of outlets thereat. The outlet has a larger cross-sectional area than the inlet. Electrolyte flow is effected in parallel to all cells and the pumping pressure is provided to supply and discharge passages communicating with the inlet and outlet, respectively, so that substantially the same operational pressure difference prevails between the supply passage and the respective discharge passage of each of the electrolyte chambers.
U.S. Pat. No. 3,905,832 to Trocciola discusses venting, but the only venting here concerned is the removal of excess air and impurities through venting.
U.S. Pat. No. 4,168,319 to Buzzelli discloses the use of a pump at the outlet to pull the electrolyte out. Here also, the cross-section of the electrolyte outlets is larger than the cross-section of the inlets. Uniformity of electrolyte supply is obtained by pumping the electrolyte out of the cell.
It is therefore an object of the invention to provide a fuel cell which forms part of a fuel cell stack to provide for an acid or electrolyte channel arrangement which permits the addition of electrolyte in the horizontal planes.
A further object of the invention is to provide for the acid addition to a fuel cell stack during operation.
Yet another object of the invention is to provide for the addition of and to the fuel cell stack of electrolyte without moving the stack from its operating position.
Another object of the invention is the provision of an acid addition arrangement which permits acid to be supplied to the matrices of a large number of fuel cells in a stack with horizontal cells without exceeding an allowable hydrostatic head which would cause flooding.